If you use them, be careful about how. Make sure the supports "just barely" contact the bottoms of the slide arms. If they are actually supporting it, the seals around the unit will break/touch/break/touch/break and so on, causing problems both immediately with wear & leakage and water damage farther on down the road. Make sure you don't forget about them before you pull in the slideout. They're good for support if someone flops down on the couch, mainly. The unit won't suddenly rock as much, even with regular stabilizer jacks down. The structures on most trailers are made to support the slideouts.

No, you do not need them. The slides were made with weight on them in mind. If there was a need for support, it would be provided (as in the rear slides of some TT's, or the Pop ups). If it did not come with a support, it does not need it.

Waste of time, money and everything else you can think of. My first fiver I purchased two to support the large slide. Trying to get them just right, not to tight not to loose was a never ending battle. To tight maybe bend something or who knows what even worse. To loose it will fall down. I still have them they make great work supports